Coating systems for automobiles normally comprise a multiplicity of coatings applied to a steel substrate. Typically, the steel is treated with a rust-proofing phosphate layer, then a cathodic electrocoat primer for additional corrosion protection is applied and cured. A layer of a primer composition is then applied and cured to smooth the surface and also to provide chip resistance to the coating system during the normal course of driving. Then one or more layers of a basecoat composition is applied, with solid color and/or flake pigments to provide the color area for the substrate. This is followed by one or more layers of a clearcoat composition, to protect and preserve the attractive aesthetic qualities of the finish on the vehicle even on prolonged exposure to the environment or weathering.
Application of the basecoat and the clearcoat compositions is normally achieved by wet-on-wet application, which means that a layer of the clearcoat composition is applied to the applied basecoat composition without curing the basecoat layer prior to the application of the clearcoat composition. The applied layer of basecoat composition may be subjected to a flash dry step prior to the application of the clearcoat. Once both the basecoat and clearcoat compositions have been applied, they are subsequently heated to a sufficient temperature to form a dried and cured finish. In the conventional method for forming the multi-layer coating film, the applied layer of primer composition is cured by heating before the application of the basecoat and clearcoat compositions. Historically, cured primer compositions have been used not only to provide a smooth surface, but also to also prevent intermixing with the overlying basecoat and avoid disrupting the appearance of the overall finish. Resistance to intermixing (sometimes referred to as “strike-in” resistance) is especially important for the appearance of basecoats containing metallic pigment flakes. Any disturbance of the orientation of the metallic pigment flakes after application over the primer composition can detract from the metallic effect of the finish. Therefore, care must be taken to ensure that the metal pigment flakes are not disturbed after application.
In recent years, it has also been strongly desired to reduce the environmental load or impact of automotive assembly plants by reducing VOC (volatile organic compounds) emissions and CO2 (carbon dioxide) emissions generated from operating painting booths and baking ovens. This has led to use of lower solvent content in the paint, increased use of waterborne coating compositions and the development of “three wet paints” which make it possible to apply the primer, basecoat and clearcoat compositions wet-on-wet-on-wet before they are cured all at once in a single bake. With this simplified application process, it can be possible to eliminate the separate primer painting booth and primer oven and to reduce the manpower needed to inspect and remove defects from the cured primer, both of which results in substantial cost savings to the automobile manufacturers. The technical hurdles of this process simplification, however, have been significant. For instance, interfacial bleeding and aesthetic appearance, as well as film properties such as chip resistance are still significant concerns and have been significant barriers to commercialization of 3-wet paint systems. Another problem with existing 3-wet paints is the need to reformulate the clearcoat to resist intermixing with the basecoat.